TURNOVER RATE OF SOIL ORGANIC MATTER AND ORIGIN OF SOIL (CO2)-C-14 IN DEEP SOIL FROM A SUBTROPICAL FOREST IN DINGHUSHAN BIOSPHERE RESERVE, SOUTH CHINA

This paper examines the carbon isotopes (C-13, C-14) of soil organic carbon (SOC) and soil CO2 from an evergreen broadleaf forest in southern China during the rainy season. The distribution of SOC delta C-13, and SOC content with depth, exhibits a regular decomposition of SOC compartments with different turnover rates. Labile carbon is the main component in the topsoil (0-12 cm) and has a turnover rate between 0.1 and 0.01 yr(-1). In the middle section (12-35 cm), SOC was mainly comprised of mediate carbon with turnover rates ranging between 0.01 and 0.025. Below 35 cm depth (underlayer section), the SOC turnover rate is slower than 0.001 yr(-1), indicating that passive carbon is the main component of SOC in this section. The total production of humus-derived CO2 is 123.84 g C m(-2) yr(-1), from which 88% originated in the topsoil. The middle and underlayer sections contribute only 10% and 2% to the total humus-derived CO2 production, respectively. Soil CO2 delta C-13 varies from -24.7 parts per thousand to 24.0 parts per thousand, showing a slight isotopic depth gradient. Similar to soil CO2 delta C-13, Delta C-14 values, which range from 100.0 parts per thousand to 107.2 parts per thousand, are obviously higher than that of atmospheric CO2 (60-70 parts per thousand) and SOC in the middle and underlayer section, suggesting that soil CO2 in the profile most likely originates mainly from SOC decomposition in the topsoil. A model of soil CO2 Delta C-14 indicates that the humus-derived CO2 from the topsoil contributes about 65-78% to soil CO2 in each soil gas sampling layer. In addition, the humus-derived CO2 contributes similar to 81% on average to total soil CO2 in the profile, in good agreement with the field observation. The distribution and origin of soil (CO2)-C-14 imply that soil CO2 will be an important source of atmospheric (CO2)-C-14 well into the future.